1. Field of the Invention
This invention is directed to floating systems, rigs and vessels used in wellbore operations; to floating systems, rigs, drill ships and vessels with a height-adjustable derrick; to methods for selectively adjusting a floating system's center of gravity; and to methods for adjusting derrick height by changing the position of part of a derrick, e.g. a crown block assembly and associated structure.
2. Description of Related Art
Well drilling has been conducted in areas where a substantial body of water overlies an oil field. In many cases a variety of fixed drill platforms mounted on legs resting on or driven into a sea floor or lake floor are used. These, however, are typically used only in relatively shallow depths of water, often not greater than about 300 feet, which is a realistic depth limit for many practical commercial operations.
Often deep water drilling is accomplished using specifically designed and constructed rigs, vessels and drill ships. Deep water and exploratory drilling has been accomplished using surface floating rigs, drilling ships or vessels which are either towed or self-propelled to a drilling site and are self-contained in that the drilling rig, auxiliary equipment, and crew's quarters form an integral part of the vessel or ship. These floating drilling systems (rigs, vessels, drill ships) are positioned over a drilling site. Certain typical rigs, vessels and drill ships have, in addition to all of the equipment normally found on a large ocean ship, a drilling platform and derrick located on the deck. In addition, such rigs, vessels and drill ships contain a hole (or “moonpool”), extending through the ship down through the hull, which allows for a drill string to extend through the ship, down into the water.
Drill ships are often used for deepwater drilling in remote locations with moderate weather environments because of their mobility and large load carrying capability. Drill ships can move from one location to the next rapidly and under their own power. On the open seas, size and height are generally not a consideration for drill ship movement; but, in certain specific circumstances, size and height limit a drill ship's mobility and can significantly increase the expense of movement from one site to another. For example, moving a drill ship through the Panama Canal can require the partial disassembly of a ship's derrick (and then its reassembly after passing through the canal) at a cost of several million dollars.
Various prior art drill ships are relatively large. For example Transocean's Discoverer Enterprise, an ultra-deepwater drill ship, is 835 feet in length and 125 feet wide and can drill a well more than 6.5 miles beneath its drill floor. Drill ships can be, in total, 20 to 30 stories high with an upright derrick over 400 feet high. The JOIDES Resolution drill ship is 470 feet long with a 202 foot high derrick.
In the past a variety of drill ship tragedies have involved the capsizing of a drill ship, particularly in stormy seas. One factor contributing to the instability of a drill ship is the height of the ship's center of gravity which is related to the height and the weight of a derrick projecting up from a ship's deck. The weight of pipe and equipment in and on the derrick can also affect the location of the ship's center of gravity. In typical drill ships, although pipe can be moved from a vertical to a horizontal position, the derrick itself is a permanent upright structure whose height is not adjustable in adverse conditions.
There are a variety of known rigs, vessels, and drill ships used in drilling and various wellbore operations; for example, and not by way of limitation, those disclosed in U.S. Pat. Nos. 2,929,610; 3,011,318; 4,064,822; 4,269,543; 4,657,438; 4,885,698; 5,139,366; 5,450,695; 5,622,452; 5,833,396; 5,906,457; 5,975,805; 5,975,806; 6,056,071; 6,047,781; 6,076,996; 6,085,851; 6,068,069; 6,443,240; 6,539,888; 6,682,265; 7,011,471; 7,163,355; 7,186,061; and U.S. Application Pub. No. 2008/0000685, and in the references cited in these patents—all these patents incorporated fully herein for all purposes.
There are a variety of known systems with a portable and/or erectable derrick or mast; for example U.S. Pat. Nos. 7,308,953; 6,860,337; 6,523,319; 5,450,695; 5,423,158; 5,342,020; 5,216,867; 4,932,175; 4,837,992; 4,757,592; 4,590,720; 4,269,395; 4,134,237; 3,996,754; 3,403,485; 3,340,938; and 2,804,949,